This invention relates in general to bearings and more particularly to a double row bearing assembly suitable for high speed operation.
An important feature of a tapered roller bearing is the ease with which it may be adjusted between conditions of end play and preload. Indeed, the adjustment is provided merely by moving the cup axially relative to the cone. While the ideal bearing setting is zero end play, that is, no end play and no preload, this setting is difficult to maintain because so many conditions influence a bearing setting. For example, the size of parts, the speed at which the bearing rotates, the external heat conditions, the type and amount of lubrication, the relative stiffness of the housing in which the bearing is located, and the nature of the fits between cone and shaft, on one hand, and the cup and housing, on the other, all have the capability of altering a bearing setting and must be considered when selecting such a setting.
End play enables the bearing to accommodate high speeds and thermal expansion of parts. Preload, on the other hand, provides maximum stability of the shaft, bearing, and housing walls. If during operation excessive preload develops, the bearing will fail. On the other hand, if excessive end play occurs, the system loses its rigidity and the fatigue life of the bearing is reduced.
It is not uncommon to mount large shafts on two double row bearings with a substantial spread between the two bearings. Each bearing in such a system (See FIG. 1) normally comprises: (1) a double cup, which is actually a single outer race having two inwardly presented tapered raceways, (2) a pair of cone assemblies, each including a cone, a complement of tapered rollers, and a cage to hold the rollers around the cone; and (3) a spacer between the two cones. The thickness of the spacer between the cones controls the adjustment for the bearing, and normally the adjustment is set for a relatively large amount of end play to accommodate thermal expansion. At high operating speeds, the cages of the double row bearings tend to fail, and these failures are normally confined to the cages for the rollers in the rows which do not have a 360.degree. load zone. It appears that failures of this nature derive from the pounding action of the rollers against their cages, and the pounding action eventually causes the cages to break at the intersections of the cage bridges and the annular rings at the large and small ends of the cage. In this regard, a radial load imposed upon a tapered roller bearing is translated into an axial component on the rollers in the load zone, that is, on the rollers through which the radial load is transmitted, and this axial component tends to expel the rollers from the end of the bearing. Actually, the axial component urges the rollers against a thrust rib at the large diameter end of the cone raceway. Not only does the thrust rib prevent expulsion of the rollers, but it further serves to properly orient the rollers between the cup and cone, for when the end face of a roller is against the thrust rib, the axis of the roller assumes a predetermined angle with respect to the face of the rib.
When either row of a double row bearing accommodates a thrust load, which in effect places that row in a condition of preload, all rollers of that row are thrust against the thrust rib and oriented thereby. The cage for that row merely serves to maintain the correct spacing between the rollers, it having no effect of the guidance of the rollers. While the rollers under preload may be guided for a full 360.degree. by their thrust rib, this is not the case with the rollers in the other row. As previously mentioned, only the rollers in the load zone are guided by the thrust rib. As these rollers leave the load zone, they tend to drift away from their thrust rib and when so disposed are guided solely by their cage. Consequently, each time a roller enters the zone of no loading for its bearing, it pounds against its cage. Furthermore, when each roller returns to the load zone, the cage must impact the roller to force it into the load zone. It is this combination of the impacts which occurs at the entrance to and exit from the load zone which eventually destroys the cage.